One thing I hope Apple doesn't do is support all the old legacy devices like they did with iOS 9. I know, people like being able to hold onto their iPad 2 for decades, but the A5 hardware is really holding developers back from taking advantage of the latest chipsets which have really evolved since 2011.
Even if a developer wanted to limit the app to JUST iOS 9, they'd still have to support the iPad 2, iPad mini, iPhone 4S, and the 5th generation iPod Touch.
Last year, after WWDC, Apple quietly discontinued the first iPad mini, and they replaced the 5th generation iPod Touch with a 6th generation one, which has an A8 chip. Now that these devices are gone, Apple's current lineup features devices with A7 processors and later. Considering devices like the iPad mini 2 skipped the A6 chip, this tells me we probably won't see THESE devices running iOS 10 either. Apple doesn't sell them anymore, and they didn't have a very long run due to them being 32-bit while their successor, the A7, was 64-bit.
So, I imagine iOS 10 will support:
iPhone 5S and later
iPad Air and later
iPad mini 2 and later
iPad Pro (both variants)
iPod Touch (6th generation)
This would probably piss a few people off, especially iPhone 5C and iPad 4 owners. But, this means that every device they support will be Retina (so we'll probably have quite a few design changes, hopefully better management of space on the home screen for the iPads which are now Retina-only), every one will be 64-bit, so they'll all support things like Metal.
Hopefully iOS 10 is more forward thinking.